Update collaborators. Refs ivanperez-keera#37.

The collaborators section contains people no longer actively
collaborating with this project.

This commit updates that section to avoid suggesting that they are
active collaborators.

[ci skip]

README.md

@@ -86,7 +86,3 @@ We would like to call on Haskell programmers, game developers and anyone with an
 ### Maintainer
 
 * Ivan Perez
-
-### Collaborators
-
-* Christina Zeller